Article pp.304-318 du Vol.23 n°2 (2003)

(1)

SCIENCES DES ALIMENTS, 23(2003) 304-318

ARTICLE ORIGINAL ORIGINAL PAPER

Behavior of dromedary milk at native and at acid pH during the ultrafiltration:

comparison with cow milk

N. Kherouatou¹, A. Dhouib², H. Attia¹*

RÉSUMÉ :

Ultrafiltration de lait de dromadaire et de son coagulum lactique : comparaison avec le lait bovin.

Le comportement du lait de dromadaire au cours d’une ultrafiltration sur membrane minérale a été étudié comparativement à celui du lait bovin. Au pH natif, le lait camelin s’est distingué par un flux nettement supérieur et par un taux de rétention en protéines nettement inférieur. L’analyse des permeats correspon- dants a révélé, d’une part, des teneurs moindres en minéraux et en citrates, ce qui suggère que la répartition de ces éléments est en faveur de la phase micellaire. D’autre part, et contrairement à ceux du lait bovin, les permeats du lait camelin se sont caractérisés par la présence de caséines. Toutes ces spécifici- tés seraient dues à des facteurs intrinsèques à ce lait (diamètre micellaire plus important, viscosité absolue plus faible, nature différente des caséines…).

L’ultrafiltration du coagulum lactique (pH 4,40) issu du lait camelin a, à l’inverse, montré un flux du permeat plus faible et un taux de rétention en pro- téines plus élevé. Ce résultat a été expliqué par l’absence d’une structure de caillé. Ce coagulum consiste, en effet, en des flocons dispersés qui provoque- raient au départ de l’opération, un colmatage interne rapide. La visualisation en statique du dépôt membranaire a permis de conforter les comportements observés en dynamique. La structure du colmatage externe est relativement aérée lors du traitement du lait camelin et est diffuse et relativement « fermée » lors du traitement du coagulum lactique correspondant.

Mots clés :

lait de dromadaire ; coagulum lactique ; ultrafiltration ; rhéologie ; microsco- pie électronique.

1. École nationale d’ingénieurs de Sfax, B.P. W, 3038 Sfax, Tunisie.

2. Centre de biotechnologie de Sfax, B.P. K, 3038 Sfax, Tunisie.

* Correspondence: E-mail: [email protected] Tel.: 216.4.274088 Fax.: 216.4.275595 10-Kherouatou (304-318) Page 304 Mercredi, 11. juin 2003 1:01 13

(2)

Ultrafiltration of dromedary milk 305

SUMMARY

The behavior of dromedary milk during the ultrafiltration on mineral membrane was studied comparatively with that of cows’ milk. At native pH, dromedary milk was marked by a better flux and a lower protein retention rate. Analysis of the corresponding permeates revealed that dromedary milk had lower mineral and citrate contents in the serum phase. In contrast with those of bovine milk, the permeates of dromedary milk were also characterized by the presence of caseins. These specificities could be attributed to the intrinsic factors of this milk (a larger average micelle size, a lower viscosity, a different structure of caseins…)

Ultrafiltration of the lactic coagulum (pH 4.40) obtained from dromedary milk has, inversely, showed a lower permeate flux and a higher protein retention rate. This result was justified by the absence of a real curd structure. In fact, this latter was made up of dispersed flakes, which induce a rapidly internal fouling at the onset of the ultrafiltration process. SEM observation of the external fouling confirmed the behaviors which were observed under dynamic conditions. Indeed, for dromedary milk, the deposit structure due to the native pH was open and the deposit structure due to the coagulum was filled and diffuse.

Key-words:

dromedary milk; lactic coagulum; ultrafiltration; rheology; electron micros- copy.

1 – INTRODUCTION

The dromedary milk does not present the requisite physical qualities to undergo the technological transformations (ABU-TARBOUSH, 1994; FARAH, 1993; FARAH et FARAH-RIESEN, 1985). It is notably unable to form a real curd structure. In fact, coagulation caused by lactic fermentation or by enzimic method did not produce a curd but simply flakes that lack firmness (ABU-TAR- BOUSH, 1994; HOSTE et al., 1985; ATTIA et al., 2001).

Numerous specificities related to that inability have already been investi- gated. Thus, even though it has a general composition similar to that of bovine milk (SAWAYA et al., 1984; BAYOUMI, 1990; ATTIA et al., 2001), the dromedary milk is characterized by a qualitatively and quantitatively different casein composition (FARAH, 1993). Moreover it represents a lower total casein content (ATTIA et al., 2001), a higher soluble casein content (ATTIA et al., 2000c), a lower k casein content (KAPPELER et al., 1998; OCHIRKHUYAG et al., 1997), different electrophoretic mobility (FARAH et BACHMANN, 1987; ATTIA et al., 2000b) and a poor rennetability (FARAH et BACHMANN, 1987).

In recent paper (ATTIA et al., 2001), we have showed that the sudden disap- pearance of the micellar state is among the possible causes of the extreme fra- gility of the dromedary milk and of its inability to be transformed into dairy

10-Kherouatou (304-318) Page 305 Mercredi, 11. juin 2003 1:01 13

(3)

306 Sci. Aliments 23(2), 2003 N. Kherouatou et al.

products. In fact, the pH interval between the points corresponding to a lot of biochemical and rheological events, 4.8-4.9 (minimum of apparent viscosity, maximum of casein solubility, maximum of casein hydration, maximum of solu- bilization of P and of citrates, maximum of buffering capacity) and those corresponding to the elaboration of the pseudo-curd, 4.8-4.7 is not enough to establish the characteristic bonds of a lactic milk curd.

Similarly, dromedary casein micelle presents a larger average diameter (BUCHHEIM et al., 1989; ATTIA et al., 2000b), a more important mineral content and citrate charge (ATTIA et al., 2000c) and a higher physical stability during acidification process (ATTIA et al., 2000c).

In the present paper, we tried to define the dromedary milk behavior during a physical separation which is very much used at present: the ultrafiltration. This study has concerned the milk at native pH and the lactic coagulum obtained from it . In fact, in dairy industries, these two products are the most treated by ultrafiltration (DAUFIN, 1998).

We have used both dynamic and static study in order to relate mass transfer with fouling microstructure. The same study has been achieved using bovine milk to visualize the effect of the original physico-chemical properties of dromedary milk.

2 – MATERIALS AND METHODS

2.1 Milk samples

Dromedary milk was a pooled milk obtained from the milking of an eighteen- dromedary herd (Camelus dromedarius) of Maghrabi breed belonging to the institute of Arid Region institute (I.R.A., Medenine, Tunisia). These samples were skimmed at 2000.g and 10°C for 15 min. This operation was repeated three times to achieve fat separation (ATTIA et al., 2000a). Cows’ milk was obtained by dispersing a skim milk powder of low-heat drying process (Laiterie du Parc, St Florent le Vieil, France) in deionized water at a same total solids as that of dromedary milk. The reconstituted milk was stored at ~ 4°C for 24 h before use.

The average composition of both milks is reported in table 1.

2.2 Coagulation

After pasteurization (63°C, 30 min), the skim milk samples were acidified with a mixed culture Lactococcus lactis ssp. lactis, L. lactis ssp. diacetylactis and L. lactis ssp.cremoris. This mother culture consists in a lactic acid starter, CH-normal 01 (Chr. Hansen’s Lab. A/S, Copenhagen, Denmark) revivified for 24 h at 25°C, in a sterilized reconstituted skim milk. After incubation (2%; w/v), both milks were incubated at 20°C ± 0.1°C until the pH reached 4.40.

(4)

2.3 Ultrafiltration

Essays of ultrafiltration were carried out on classic-cross-flow filtration installation as described by ATTIA et al. (1988). The following conditions were used: average transmembrane pressure, 400 kPa; tangential flow velocity, 3 m.s^-1; temperature, 50˚C; volume concentration factor (FCV),1.

2.4 Membranes

Alumina microfiltration membranes (Membralox, SCT, Bazet, France) with an average pore diameter of 0.2 µm were used. The dynamic study was carried out on a multichannel filter element (19 channels) and the static study on tubular sections with usable length of 8 cm, i.d. 1.5 cm.

2.5 Method of obtaining fouling under static conditions

Milk samples were put into contact with the tubular lengths of membrane using the fouling technique described by VÉTIER et al. (1986). After 60 min contact at 50°C, the tubes were emptied and the fouling that remained at the surface of the filter layer was observed under a scanning electron microscope.

2.6 Scanning electron microscopy (SEM)

Samples of the fouled membranes (0.5 cm²) taken from the central part of the tubes were prepared using the technique described by ATTIA et al.

(1991a) and examined with a Philips XL 30 (Philips, Limeil Brevannes, France) Table 1

Comparison of chemical composition of dromedary skim milk and reconstituted cows’ skim milks (χ: mean values of three replicates, SD: standard deviation)

Composition

Average contents (g.kg^-1)

Dromedary Cow

χ SD χ SD

Total solids Lactose Total nitrogen Caseins Whey proteins NPN^* Fat Ash Calcium Magnesium Phosphorus Citrates

96.07 54.03 30.72 20.60 7.55 2.57 1.20 9.92 1.22 0.10 1.02 1.85

2.00 1.12 0.64 0.42 0.22 0.02 0.20 0.12 0.03 0.01 0.02 0.04

96.02 54.70 32.95 26.30 4.90 1.75 1.20 9.46 1.30 0.10 1.12 1.68

1.90 1.00 0.48 0.30 0.15 0.01 0.10 0.10 0.02 0.01 0.01 0.02

* non protein nitrogen

(5)

after drying to CO₂ critical point on a Baltec CPD O30 apparatus and gold coating on Baltec MED 20 apparatus (Balzers Union, Balzers, Germany).

2.7 Rheology

The flow curves of the milk samples were carried out using a StressTech Reologica rheometer (Reologica Instruments AB, Lund, Sweden) with a coaxial cylinders measuring system (inner diameter 25 mm and outer diameter 27 mm) and at 50˚C ± 0.1˚C. A volume of 15.9 mL of milk sample was transferred from the feeding tub back to the rheometer measuring system just before the beginning of the operation of ultrafiltration.

2.8 Chemical analyses

– Calcium (Ca) and Magnesium (Mg) were measured by an atomic absorp- tion apparatus (Z-6100, Hitachi Instruments Engineering Co., Ibaraki–ken, Japan) in the presence of lanthanum chloride (Sigma Chemical Co., St Louis, USA).

The concentration of phosphorus (P) was determined by a colorimetric method with ammonium molybdate (PIEN, 1969).

– Citrate content was determined enzymatically using a commercially availa- ble test kit (Boehringer Mannheim, Germany, catalog number 139 076).

– Total Nitrogen (TN), non-protein nitrogen (NPN) and non casein nitrogen (NCN) were achieved using Kjeldahl method (Afnor, 1993) with a Büchi apparatus (425 and 320) (Büchi Laboratoriums, Flawil, Switzerland).

Casein content was calculated by the difference between TN content and NCN content after separation according to ROWLAND (1938).

– Total solids, ash, lactose and fat contents were determined according to standard methods (Afnor, 1993), respectively, by drying at 102-104°C, by incineration at 550°C, by Bertrand method and by extraction with a sox- hlet system.

3 – RESULTS

3.1 Study of the processing of milks at native pH

Variation of flux ultrafiltrate and protein retention rate related to time are given in figure 1. These two parameters tended to stabilize rapidly (~ 5 min).

However, at this state of equilibrium, dromedary milk showed a higher performance (96 L/h.m²vs 74 L/h.m²) and a lower efficiency (94.5% vs 99.6 %).

Protein loss was followed during ultrafiltration (figure 2). Since the start of fluxes stabilization, dromedary milk permeates contained whey protein and caseins, in a proportion of ~ 2/3 and ~ 1/3 respectively. The permeates of cow’s milk were characterized by the absence of caseins.

(6)

As related to ultrafiltration time (figure 3), electrolytes contents of permeates were practically constant for both types of milk. The permeates of dromedary milk presented lower contents of Mg, 0.033 g/kg (SD = 0.001) vs. 0.06 g/kg (SD = 0.002), of P, 0.34 g/kg (SD = 0.02) vs 0.44 g/kg (SD = 0.01) and of citrate,1.27 g/kg (SD = 0.03) vs 1.53 g/kg(SD = 0.03) than bovine milk. On the contrary, there was no difference of Ca content in the two milks.

3.2 Study of the processing of lactic coagulums (pH 4.4)

The permeate flux (figure 4) was lower for the dromedary milk coagulum than bovine milk coagulum. The gap was close to 55 L/h.m² at equilibrium state.

The protein retention rate of this coagulum was lower at the onset (79 % vs 97 %) then it increased rapidly and became higher than that of bovine milk coagulum (99.5 % vs 98.5 %).

50 60 70 80 90 100 110 120 130 140 150

0 20 40 60 91

92 93 94 95 96 97 98 99 100

Permeate flux (L, h-1.m-2) Protein retention (%)

t_f(min) Figure 1

Change in permeate flux ( , ) and protein retention rate ( , ) with filtration time for dromedary skim milk (filled symbols) and cows’ skim milk (open symbols).

Experimental conditions: 50˚C; 3 m/s; 0.4MPa; FCV=1.

Error bars indicate the maximum SD of three replicates.

(7)

3.3 Rheological study

Figure 5 gave absolute viscosity at 50°C for both kinds of milk. Dromedary milk seemed relatively less viscous (0.66 mpa.s vs 0.82 mpa.s).

Examination of the rheogramm (shear stress-shear rate) showed that lactic coagulums obtained from these two kinds of milk displayed two successive types of behavior. At the onset, the coagulum behaved as a pseudoplastic fluid with apparent viscosity decreasing with increasing shear rate. At the second stage, the material behaved as a Newtonian fluid and its viscosity remained constant. This linear behavior was obtained more rapidly for dromedary milk coagulum (20 s^-1vs 130 s^-1).

40 50 60 70 80 90 100

0 10 20 30 40 50 60

t_f(min)

Sp/Tp (%)

Figure 2

Protein loss in the permeates of dromedary skim milk ( ) and cow’s skim milk ( ) as a function of filtration time.

Sp : Quantity of soluble protein in the permeate;

Tp : total proteins content in the permeate.

(8)

3.4 Sem observation of external static fouling

The surface of the deposit on the membrane is shown in figure 6. Fouling induced by milks (native pH) appeared to consist of aggregated casein micelles, which gave a dense structure for bovine milk (figure 6a) and an “open” structure for dromedary milk (figure 6b). Deposit edified by the lactic coagulums (pH 4.4) consists of a porous network for bovine milk coagulum (figure 6c) and for dromedary milk coagulum, a diffuse structure which was strongly adsorbed at the membrane surface (figure 6d).

0,3 0,5 0,7 0,9 1,1 1,3 1,5 1,7

0 20 40 60 0,02

0,03 0,04 0,05 0,06 0,07

t_f(min)

Ca, P, Citrates (g.kg-1) Mg (g.kg-1)

Figure 3

Change in Ca ( , ) ; Mg ( , ); P ( , ); and citrates ( , ), contents in the dromedary skim milk permeate (filled symbols)

and in the cows’ skim milk (open symbols).

Values are the means of three replicates.

(SD of some values are indicated in the text).

(9)

4 – DISCUSSION

At native pH, the development of both milks ultrafiltration was characterized by a rapid build up of external membrane fouling as shown by the stabilization of permeate flows and that of protein retention (figure 1).

Since the global composition of dromedary milk was very similar to that of bovine milk (table 1), the deposit edified by this milk must have the same nature as that of bovine milk i.e. essentially proteins (VETIER et al.,1986; ATTIA at al., 1991b).

We have already reported that dromedary milk micelles at initial pH had a structure similar to that of bovine milk micelles (ATTIA et al., 2000c). Indeed by SEM, they appeared in a spherical shape with different sizes.

0 50 100 150 200 250 300 350 400 450 500

0 20 40 60

75 85 95

Permeate flux (L.h-1.m-2) Protein retention (%)

t_f(min) Figure 4

Change in permeate flux ( , ) and protein retention rate ( , ) with filtration time for dromedary lactic coagulum (filled symbols) and cows’ lactic coagulum (open

symbols). Experimental conditions: 50˚C; 3 m/s; 0.4MPa; FCV=1.

(10)

The greater performance and the poor efficiency presented by dromedary milk could consequently only be attributed to its structural and physicochemical specificities. Thus, significant differences between cow and dromedary milk in their primary structures were reported by OCHIRKHUYAG et al. (1997) et KAP- PELER et al. (1998). In fact, variations in physico-chemical properties of casein must be taken to account for the development of fouling structure (ATTIA et al., 1991a).

Similarly, the lower absolute viscosity of dromedary milk (figure 5) implies a better mass transfer (figure 1) in agreement with poiseuille law. The higher size of dromedary milk micelles could explain the lower absolute viscosity. As a result, of the reduction in the suspended particles number, micelle volume frac- tions will decrease leading to limited contacts or friction forces between proteins. At this level, it is worth noticing that by carrying out direct measurements

0 5 10 15 20 25

0 100 200 300

0,5 0,6 0,7 0,8 0,9

ηapp, coagulum (mPa.s) ηabs, milk (mPa.s)

Shear rate (s^-1) Figure 5

Change in viscosity with shear rate for the studied skim milks ( , ) and for their lactic coagulum ( , ) sampled just before the beginning of the ultrafiltration

process. Filled symbols: dromedary. Open symbols: cow.

happ.: apparent viscosity ; habs.: absolute viscosity.

(11)

on the screen of the electron microscope on 800 particles, we have estimated to be 2/3 the proportion of micelles presenting a size between 0.35 µm and 0.5 µm (ATTIA et al., 2000b).

Membrane fouling was assessed visually using scanning electron micros- copy (figure 6). Thus, static external fouling due to dromedary milk showed larger pores (figure 6b). This loose microstructure is in agreement with the higher ultrafiltration performance observed for this milk (figure 1). It also justifies the presence of caseins in the dromedary milk permeates and their absence in those of bovine milk up to equilibrium state, i.e. t_f> 5 min (figure 2). In fact, the deposit of juxtaposed micelles induced by bovine milk was compact and then prevented micelles from infiltrating filter layer (figure 6a).

The quantitative study of mineral plus citrate charges of permeates (figure 3) permitted to reveal their partition between micellar and serum phases. In fact, the mineral charge of permeates reflected the mineral charge of milk soluble phase, as demonstrated for bovine milk by DAVIES and WHITE (1960) or BRULÉ et al. (1974) during skim milk ultrafiltration.

a b

c d

Figure 6

SEM micrographs of external fouling obtained after contact (50°C; 60 min) under static conditions of the 0.2 µm mineral membrane with cows’ skim milk (a),

dromedary skim milk (b), cows’ lactic coagulum (c) and dromedary lactic coagulum (d).

(12)

The present results obtained for dromedary milk consolidate those we have already reported in a previous study when the separation of milk phases was achieved using another separation process – the ultracentrifugation (ATTIA et al., 2000c).

In fact, Mg, P and mainly citrates were found to be involved to a more important extent in the micellar phase, about 2/3, 2/3, 1/3 respectively versus about 2/5, 3/5, and 1/10 for bovine milk. This higher mineral content would be related to the larger micelle size of dromedary milk since it is well known that, in bovine milk, large micelles contain more saline bridges than small ones (BRULÉ et al., 1997; DALGLEISH et al., 1989).

The ultrafiltration of dromedary milk coagulum gave similar curves to those reported for cows’ milk coagulum (ATTIA, 1988; 1991a and b; MAHAUT et al., 1982). However, dromedary milk coagulum showed a very distinct value of per- formance and efficiency (figure 4).

This result was expected since lactic coagulum of dromedary milk did not present a curd structure as that of bovine milk but consisted of small casein flakes that were dispersed in mellow phase (ATTIA et al., 2001). We think that the great dispersion of these casein flakes induced a very rapid and extensive internal fouling and an important obstruction of the filter channels.

This phenomenon would explain the very important rise in protein retention rate during the first 10 min of operation (figure 4). Afterwards, these small parti- cles were strongly adsorbed to the membrane surface and completely coated its luminosity. This phenomenon led to a diffuse deposit (figure 6d) which was easily compactable by transmembrane pressure. This filled deposit suggested that the possibilities of aggregation between the casein particles of dromedary milk were very reduced if not absent. It is very probable that the rapid transition of dromedary milk from the liquid state to the gel state (ATTIA et al., 2001) would not permit establishment of hydrophobic, hydrogenous and nonmineral electrostatic bonds which characterize the lactic curd of bovine milk.

Change in the apparent viscosity during shear rate increase (figure 5) con- firmed that dromedary milk coagulum, easily, disperses into microscopic particles of casein. In fact, the Newtonian behavior was obtained very rapidly (20 s^-1) which reflects the very fast orientation of these particles in the flow direction hence the rapid fall in viscosity (figure 5).

The general rheological behavior of the bovine milk curd was similar to that of dromedary milk pseudo-curd. However, its flow became Newtonian only much later (130 s^-1). This means that the bonds forming bovine milk curd appeared to be stronger and more numerous. Higher shear forces and veloci- ties were then required to induce the flow of bovine milk curd.

5 – CONCLUSION

In this work, we tried to identify ultrafiltration characteristics of both dromedary milk and dromedary milk coagulum, without concentration.

(13)

The revealed specifities were attributed to inherent factors in dromedary milk (absolute and apparent viscosity, micellar size, mineral fraction partition…).

As in the case of ultrafiltration of bovine milk, relationships exist between product composition, membrane fouling, rheological properties and mass transfer through the filter layer.

Indeed a higher protein retention rate was observed during the ultrafiltration of dromedary “pseudo-curd”, we intend to search for optimal operating conditions to concentrate it using this physical process. The chosen experimental design should take into account the physical and biochemical properties recently revealed for this milk. We can then elaborate, for example, fresh white cheese the manufacture of which is very difficult because dromedary milk does not produce a real curd able to undergo the draining stage.

Obviously, the fermentation of dromedary milk concentrated beforehand by ultrafiltration, is not conceivable. In fact, the buffering capacity of dromedary milk was higher than that bovine milk mainly for low pH (ATTIA et al., 2001).

Moreover, dromedary milk micelle is more mineralized than its homologous in bovine milk (ATTIA et al., 2000c). The release of its colloidal minerals to the sol- uble phase of the concentrated lactic retentate will be very important.

ACKNOWLEDGEMENTS

We thank Fakhfakh Zouhair (USCR Microscopie, Faculté des sciences, 3000 Sfax) for SEM observations.

REFERENCES

ABU-TARBOUSH H.M., 1994. Growth beha- viour of Lactobacillus acidophilus and bio- chemical characteristics and acceptability of acidophilus milk made from camel milk.

Milchwissenschaft, 49, 379-382.

AFNOR, 1993. Contrôle de la qualité des produits alimentaires. Lait et produits lai- tiers, Afnor (ed.), Paris.

ATTIA H., BENNASAR M., TARODO DE LA FUENTE B., 1988. Ultrafiltration sur membrane minérale des laits acidifiés à divers pH par voie biologique ou chimique et de coagulum lactique. Lait, 68, 13-32.

ATTIA H., BENNASAR M., TARODO DE LA FUENTE B., 1991a. Study of the fouling of inorganic membranes by acidified milks

using scanning electron microscopy and electrophoresis. I. Membrane with pore diameter 0.2 µm. J. Dairy Res., 58, 39-50.

ATTIA H., BENNASAR M., TARODO DE LA FUENTE B., 1991b. Study of the fouling of inorganic membranes by acidified milks using scanning electron microscopy and electrophoresis. II. Membrane with pore diameter 0.8 µm. J. Dairy Res., 58, 51-65.

ATTIA H., KHEROUATOU N., FAKHFAKH N., KHORCHANI T., TRIGUI N., 2000a. Dro- medary milk fat: Biochemical, microscopic and rheological characteristics. J. Food Lipids, 7, 95-112 .

ATTIA H., KHEROUATOU N., TRIGUI N., KHORCHANI T., 2000b. Mise en évidence 10-Kherouatou (304-318) Page 316 Mercredi, 11. juin 2003 1:01 13

(14)

des propriétés rhéologiques et structura- les de la fraction colloïdale du lait de dro- madaire. Microb. Hyg. Aliment., 34, 18-24.

ATTIA H., KHEROUATOU N., NASRI M., KHORCHANI T., 2000c. Characterization of the dromedary milk casein micelle and study of its changes during acidification.

Lait, 80, 503-515.

ATTIA H., KHEROUATOU N., DHOUIB A., KHORCHANI T., 2001. Dromedary milk fermentation: Microbiological and rheological characteristics. J. Ind. Mic., 26, 263-270.

BAYOUMI S., 1990. Studies on composition and rennet coagulation of camel milk. Kiel Milch. Forsch, 42, 3-8

BRULÉ G., MAUBOIS J.L., FAUQUANT J.

1974. Etude de la teneur en éléments minéraux des produits obtenus lors de l’ultrafiltration du lait sur membrane. Lait, 54, 600-615.

BRULÉ G., LENOIR J., REMEUF F. 1997. La micelle de caséine et la coagulation du lait. In: ECK A., GILLIS J. C. (ed.), Le fro- mage, 7-41, Éditions Tec. et Doc., Paris.

BUCHHEIM W., LUND S., SCHOLTISSEK J., 1989. Vergleichende Untersuchungen zur Struktur und Größe von Casein Micellen in der Milch verschiedener Species. Kiel Milch. Forsch, 41, 253-265.

DALGLEISH D. G., HORNE D. S., LAW A. J. R., 1989. Size-related differences in bovine casein micelles. Biochim. Biophys. Acta, 991, 383-387.

DAVIES D.T., WHITE J.C.D., 1960. The use of ultrafiltration and dialysis in isolating the aqueous phase of milk and in determining the partition of milk constituents between the aqueous and disperse phases.

J. Dairy Res., 27, 171-190.

DAUFIN G., 1998. Les séparations par membrane dans les procédés de l’industrie lai- tière. In: DAUFIN G., RENÉ F., AIMAR P.

(ed.), Les séparations par membrane dans les procédés de l’industrie alimentaire,

282-371, Éditions Tec & Doc, Paris.

FARAH Z., 1993. Composition and characte- ristics of camel milk. J. Dairy Res., 60, 603-626.

FARAH Z., FARAH-RIESEN M., 1985. Separa- tion and characterization of major compo- nents of camel milk casein.

Milchwissenschaft, 40, 669-671.

FARAH Z., BACHMANN M.R., 1987. Rennet coagulation properties of camel milk. Mil- chwissenschaft, 42, 689-692.

HOSTE C, PEYRE DE FABREGUES B., RICHARD D., 1985. Le dromadaire et son élevage. Institut d’élevage et de médecine vétérinaire des pays tropicaux, Maison Alfort, France.

KAPPELER S., FARAH Z., PUHAN Z., 1998.

Sequence analysis of Camelus dromeda- rius milk casein. J. Dairy Res., 65, 209- 222.

MAHAUT M., MAUBOIS J.L., ZINK A., PAN- NETIER R., VEYRE R., 1982. Manufacture of fresh cheese by ultrafiltration of curd.

Tech. Lait, 961, 9-13.

OCHIRKHUYAG B., CHOBERT J.M., DALGA- LARRONDO M., CHOISET Y., HAERTLÉ T., 1997. Charaterization of caseins from Mongolian Yak, khainak and bactrian camel. Lait, 77, 601-613.

PIEN J., 1969. Dosage du phosphore dans le lait. Lait 49, 175-188.

ROWLAND S.J., 1938. The determination of the nitrogen distribution of milk. J. Dairy Res., 9, 42-46.

VETIER C., BENNASAR M., TARODO DE LA FUENTE B., 1986. Study of the interac- tions between milk constituents and mine- ral membranes for microfiltration. Lait, 66, 269-287.

SAWAYA W.N, KHALIL J.K., AL–SHALHAT A., AL–MOHAMMED H., 1984. Chemical composition and nutrition quality of camel milk. J. Food Sci., 49, 744 -747.

(15)